Sunlight contains all visible wavelengths. The sky is blue because the atmosphere scatters some wavelengths more than others.
The scattering
Air molecules are much smaller than the wavelength of visible light. When light encounters particles that small, the scattering is strongly wavelength-dependent – shorter wavelengths scatter far more than longer ones.
Blue and violet light scatters in all directions. Red light mostly continues straight through.
So when you look at the sky away from the sun, you are seeing scattered blue light arriving from every direction.
Why not violet
Violet scatters even more than blue, so the obvious question is why the sky is not violet.
Two reasons. The sun emits less violet than blue. And human eyes are considerably less sensitive to violet – our colour vision peaks elsewhere. The combination makes the sky read as blue.
Why sunsets are red
Same mechanism, different geometry. When the sun is low, its light travels through far more atmosphere to reach you.
Along that longer path, nearly all the blue has been scattered away. What arrives directly is what was left – the reds and oranges.
Why some sunsets are more dramatic
Larger particles – dust, smoke, pollution – scatter differently and can intensify the colours. Which is why sunsets after volcanic eruptions or during wildfire seasons are unusually vivid.
Why the sky is black in space
No atmosphere, no scattering. The sun is bright and the sky around it is dark, which is exactly what photographs from orbit show.